Minimally-invasive cardiac output (CO) monitoring to follow changes in CO would be helpful in anaesthesia practice. Two Doppler systems marketed for this purpose include the CardioQ (Deltex Medical Group, Chichester, United Kingdom), which uses an oesophageal probe, and the USCOM (USCOM Ltd., Sydney, NSW, Australia), which uses a hand-held probe. The aim of the study was to assess the ability of these two methods to track CO during major surgery and to determine their relationship. Twenty patients, age 58 (26 to 81) years, (m/f) 15/5, requiring abdominal surgery were studied. The surgical procedures lasted between 128 and 408 minutes and a total of 285 data pairs (8 to 22 per case) were collected. Time plots showed good tracking ability across a wide range of CO in most patients. Correlation between the two devices was excellent in 14 patients (R-2 >0.85), good in another four (R-2 >0.64) and poor in two. Regression line data supported the hypothesis that CardioQ under-reads at low CO and over-reads at high CO in respect to the USCOM. However, the precision between the two CO readings was poor with wide limits of agreement and a percentage error of +/- 37%. These findings indicate that these devices individually track changes in CO in many patients but cannot be relied upon to provide the same values.